As is well known, any oscillographic instrument must surely include a cathode-ray tube (CRT) which is the output or display section. A cathode-ray tube generally consists of a triode section for furnishing a controllable source of electrons to a focusing section which focuses or forms the electrons into an electron beam. The electron beam is then deflected both vertically and horizontally in a deflection section and may be accelerated in an acceleration section to strike a phosphor-covered screen section with enough velocity that light is emitted by the phosphor.
As an apparatus for the deflection of an electron beam within a cathode-ray tube, though not exclusively, a typical conventional system may include a meander line of conductive material and an oppositely disposed ground electrode. When the meander line is excited, an electric deflection field is produced between the line and the ground electrode. If the phase velocity of the electric field traveling along the optical axis of the tube coincides or synchronizes with the velocity of the electron beam passing through the deflection field, the electron beam is deflected proportional to the strength of the electric field. However, the phase velocity of the electric field changes with an increase of the operating frequency so that the synchronization between the phase velocity and the velocity of the electrons cannot be maintained at a high operating frequency. This is, of course, a severe limitation of the deflection apparatus as it becomes impossible to observe signals on the screen of the cathode-ray tube which have frequencies higher than a certain limit value.
To overcome the disadvantage of the above described frequency limitation, there are systems which utilize a delay line type of deflection apparatus. For example, in U.S. Pat. Re. No. 28,223 to Odenthal et al there is described a deflection apparatus which includes a pair of helical deflector members having rectangular turns each having a pair of flat side portions separated by a deflector portion of different width. The system also includes two pairs of grounded adjustable compensator plates which are positioned adjacent the flat side portions on opposite sides of both helical members to form delay lines of substantially uniform characteristic impedance. This system therefore reduces the deflection signal velocity in the axial direction along the helical deflector until it is equal to the electron beam velocity to enable very high frequency signals to deflect the electron beam without appreciable distortion. However, this deflection apparatus requires that the compensator plates adjacent the flat opposite sides of both helical deflectors be precisely maintained in that to maintain the proper or desired characteristic impedance, the spacing from the deflectors is critical thereby complicating the construction techniques, as well as providing a structure which gives a rather weak and uneven surface to the beam side of the structure.
In others of these apparatus, for example U.S. Pat. Nos. 3,376,464 to Loly et al, 3,670,196 to Smith, 3,849,695 to Piazza et al, etc., there are described systems which give an adequate deflection of the beam for signals of a fairly high frequency, yet these apparatus permit the existance of a dispersion of the phase velocities which cannot be neglected.